In ground-based microwave communication or mobile communication of recent years, a signal having a large difference between peak power and average power is often used. In this case, a difference between saturation output power of an amplifier configured to amplify a signal and the peak power of the signal is increased. Thus, signal distortion is increased, and operation efficiency of the amplifier is reduced. In view of this, in order to operate the amplifier at high efficiency, a load modulation amplifier typified by a Doherty amplifier is sometimes used as disclosed in Patent Literature 1, for example.
The Doherty amplifier generally divides a high frequency input signal that has been input into two signals. One of the divided signals is input to a carrier amplifier, and the other signal is input to a peak amplifier while being provided with a phase delay of 90 degrees, that is, ¼λ. Each of the carrier amplifier and the peak amplifier includes matching circuits for obtaining impedance matching on the input side and the output side. The carrier amplifier operates with, for example, an A-class or AB-class bias to always amplify the input signal. Meanwhile, the peak amplifier operates with a C-class bias to amplify the input signal of predetermined power or more. The signals that have passed through the two amplifiers each pass through an impedance converter of a combination circuit unit, and then are combined. The combination circuit unit further includes a ¼ wavelength phase delay line configured to provide a phase delay of 90 degrees to the output on the carrier amplifier side before combination. The combined signal is output via the impedance converter.
The Doherty amplifier ideally has output power at which the maximum drain efficiency is obtained at two positions, and hence an output power range with high drain efficiency can be increased. Therefore, it can be said that the Doherty amplifier is one effective method for achieving a high efficiency characteristic in a system in which a signal having a large difference between peak power and average power is used.
The Doherty amplifier is described in detail later.